maintainers: add stephen-huan

maintainers/maintainer-list.nix

···
     githubId = 89950;
     name = "Stéphan Kochen";
   };
+  stephen-huan = {
+    name = "Stephen Huan";
+    email = "[e-mail]";
+    github = "stephen-huan";
+    githubId = 20411956;
+    keys = [{
+      fingerprint = "EA6E 2794 8C7D BF5D 0DF0  85A1 0FBC 2E3B A99D D60E";
+    }];
+  };
   stephenmw = {
     email = "[e-mail]";
     github = "stephenmw";